MESOZOIC-CENOZOIC TECTONIC EVOLUTION OF SW JAPAN: REVALIDATION OF THE COLLISIONAL MODEL

The Japanese Islands are often wrongly considered as a typical example of accretionary orogen. The model of “Pacific-type orogeny” of the 70s, emphasizing the purported absence of nappes, has been questioned in the 80s, with the discovery of the actual structure made of a pile of large thrust sheets and the proposal of collisional models, involving the intermittent underthrusting of buoyant blocks like micro-continents.

However, the revised plate tectonic interpretation of the history of the Japanese islands, considering new ocean plate stratigraphy and radiometric dating of metamorphic units acquired since the 1990s, is again presented by Japanese scientists in terms of accretionary processes linked to a steadily oceanic subduction, with an episodic ridge subduction: the so-called “Miyashiro-type orogeny”.

The review of dataset leads to the following conclusions. The structure of SW Japan is made of a pile of polydeformed sub-horizontal nappes, similar to the one encountered in collisional orogens. The geodynamic mechanisms advocated for the tectonic building within the accretionary orogeny concept are inappropriate. A permanent oceanic subduction with the intermittent subduction of an active ridge or seamount chain is unable to build such structures, as it induces in fact an acceleration of the tectonic erosion; the underthrusting of a micro-continent or mature arc is likely needed. The exhumation of Sanbagawa HP schists suggests the setting of a continental subduction. The petrological and new geochemical data strongly support the existence, beneath the nappes of accretionary complexes, of continental bodies showing affinities with South China, from which they were once separated. The episodic underthrusting of such blocks was responsible for the tectonic piling. Tectonic erosion plaid likely a major role in removing material during the intervening subduction stages.

A revised geodynamic model, implying the collision of buoyant blocks, is proposed for explaining the orogenic crises which took place respectively at around 240, 130, 80-60, and 20 Ma ago in SW Japan.